The invention relates to the field of tribological contacts, and more generally to any system where a pin is tightly assembled and is submitted to stress along a preferential direction inducing a stress of said pin according to a partial angular sector. The invention advantageously relates to a piston pin.
Two types of piston pin-to-connecting rod connections are currently used:
a) Floating connection
b) Tight connection
A floating connection enables to assemble the pin so that it can freely rotate in the connecting rod and in the piston. The connecting rod and the piston may in certain cases be bushed by means of a shrunk-on brass piece. The pin is stopped in translation by means of snap rings (circlipses). The snap rings are held in position in grooves machined at the ends of the piston pin opening. The floating assembly enables to substantially decrease the relative speed between the bodies in contact and to thus push back the seizing limit while decreasing contact friction. This assembly thereby has a higher performance. It however requires a precise machining of the connecting rod (or ring) as well as the machining of recesses to block the snap rings. The snap rings also have to be mounted on the assembled piston-pin-connecting rod assembly, which requires using specific means and tools. The piston compression height is constrained by such an assembly and does not enable to optimize the piston mass. It is thus preferentially used in high-load applications (diesel or turbo gasoline engines). Coated pins (with a DLC coating, for example) enable to push back seizing limits while decreasing the friction (see FIG. 11). According to the application, the CO2 gain may reach from 0.2 to 0.5% on a NEDC cycle (according to the application).
A tight connection does not require using snap rings. The pin is tightly assembled in the connecting rod by a connecting rod heating process. Rapid heating means (induction) are generally used on assembly lines. The pin thus tightened follows the pendular motion of the connecting rod which imposes thereto an alternating rotating motion in the piston pin holes. Such a relative motion makes the assembly more sensitive to pin seizing phenomena in the piston. It is thus necessary to provide a good lubrication thereof (for example, by means of recesses and/or grooves) and the use of a coating (DLC, for example) enables to increase the resistance to seizing while decreasing friction. The connecting rod machining is extremely simplified and the absence of a bushing enables to decrease the cross-section of the small end of the connecting rod to minimize the bulk thereof, in terms of outer diameter as well as of width. This technology enables to optimize the piston compression height and provides a mass gain for certain applications. It substantially contributes to decreasing the height of engines. On small vehicles, this solution enables to decrease the height under the hood and to thus limit the aerodynamic coefficient, and accordingly aerodynamic losses. Such a technology is reserved to low-load applications, which however represents most of urban or medium-range vehicles. The mass gain may also enable to limit hammer effects and enable to suppress balance shafts, particularly on 2 or 3-cylinder applications. The potential gains thus have to be expressed in the context of the applications.
The applicants reckon that from 30% to 40% of engines produced to date use a tight connection. The use of a DLC coating for tight pins provides a significant gain on friction, mass, and seizing resistance. FIG. 12 is a perfect illustration thereof. For a same application, the use of DLC enables to decrease the pin mass by 20% and to increase the specific pressure by 10%.
Piston pins provided with an antifriction coating are known, for example, from patents DE 10 2006 008 910 A1, U.S. Pat. No. 6,886,521 B2, and U.S. Pat. No. 7,228,786 B2. Some constructors already use this serial manufacturing technology. However, the application of such friction-decreasing coatings is not economically possible if the deposition rate is low due to the deposition method. In the case of tight pins, it is difficult to generalize this solution, due to its cost but also due to the programmed wearing of the coating used.
The applicants have studied these applications and have observed an atypical behavior of tight coated pins with respect to floating coated pins. In the case of a tight pin, the wearing is concentrated on the upper area of the pin, while the lower area is simply polished. After endurance tests, it can be observed that the DLC coating may have totally disappeared from the upper contact area while the coating thickness remains almost intact. FIG. 12 shows the results obtained after an endurance test equivalent to 300,000 km on a vehicle. These results perfectly illustrate this phenomenon. An explanation of the phenomenon can be provided by studying the forces exerted on the pin (see FIGS. 14 and 15).
Based on this observation, the applicants have devised a solution to create a pin having different properties according to its orientation.
The desired aim according to the invention is to:                1°) Decrease the friction in the pin/housing contact by using a DLC coating on the pin.        2°) Adapt the type of DLC coating to the tight pin application in order to increase the resistance to wear without damaging the antagonist.        3°) Create a variable distribution of the coating corresponding to the preferential load area of tight pin applications.        4°) Decrease the treatment time for the DLC coating.        5°) Increase the loading capacity of coating enclosures.        6°) Simplify the pin positioning means used for the coating of the pins used up to now, by for example suppressing the rotation.        7°) Increase the loading capacity of coating machines.        8°) Increase the pin resistance by means of an asymmetrical shape corresponding to the polar diagram of the applied forces.        9°) Enable to assemble the tight pins with existing assembly means. The pins just have to be properly oriented on assembly.        
The present invention aims at providing a generic structural unit where the coated piston pin can be formed by using a rather simple technology.
More generally, the problem that the invention aims at solving is, in the case of a tightly-assembled pin stressed according to a preferential direction, thus inducing a stress on the pin according to a partial angular sector, which particularly concerns piston pins, to enable the engine operator to increase the load of the rotating assembly, to suppress any risk of seizing by using a DLC-type coating, particularly in special conditions, to significantly decrease implementation costs.